A parking lock actuating device and a driving gear selector device for an automatic transmission of a motor vehicle are known from DE 102 45 386 A1. The parking lock actuating device is made with a parking lock cone by means of which the parking lock can be engaged and disengaged. In one of the embodiments of the parking lock actuating device shown, one end of the parking lock cone is connected to a lever mounted to swivel on a bearing. When the lever is swiveled about the bearing the parking lock is displaced in the axial direction. The lever is connected by a rod to an actuating piston or piston unit, which is arranged to move axially in a cylinder. Between the actuating piston and an end of the cylinder facing toward the bearing is arranged an engagement energy storage device in the form of a spring, or parking lock engaging spring. The parking lock engaging spring exerts a force on the actuating piston, which force is directed away from the bearing.
In addition the parking lock actuating device comprises a release device. For this, a release piston is also arranged in the cylinder. Between an end of the cylinder opposite the end facing the bearing and the release piston is arranged a locking release energy storage device in the form of a parking lock release spring. The parking lock release spring exerts a force on the release piston in the direction of the bearing and pushes the release piston in opposition to a retaining device in the form of a lifting magnet which comprises an electromagnetically actuated retaining pin.
A pressure chamber formed by the release piston and the cylinder is connected by a pressure line to a hydraulic system of the automatic transmission. A parking lock pressure is present in the pressure chamber. By means of a valve the connection to a pump powered by a drive engine of the motor vehicle can be formed or interrupted. If the connection is formed and the working pressure produced by the pump is high enough, then a force acts on the actuating piston in the direction of the bearing, which moves the actuating piston in opposition to the force of the parking lock engaging spring, toward the bearing. The lever then swivels counter-clockwise and the parking lock is disengaged.
If the parking lock pressure falls below a limit value, the force exerted by the parking lock engaging spring on the actuating piston is larger than the force acting in the opposite direction. Thus, the actuating piston moves away from the bearing so the lever swivels clockwise. The parking lock is thereby engaged. The parking lock pressure can be reduced to below the limit value by interrupting the connection between the pressure chamber and the hydraulic system and at the same time connecting the pressure chamber to a tank (not shown). In addition the parking lock pressure can be reduced by reducing the working pressure, for example by switching off the drive engine. Thus, it is not possible to disengage the parking lock without the release device if the working pressure is no longer high enough.
If the parking lock has to be disengaged when the working pressure is insufficient, the lifting magnet is actuated by the control unit in such manner that the retention of the release piston is released and the release piston is moved by the parking lock release spring in the direction toward the bearing. The size of the parking lock release spring is chosen such that the release piston moves, encounters the actuating piston, and carries the latter along with it. Thus the lever swivels counter-clockwise and the parking lock is released.
If the parking lock pressure is increased again, the release piston is moved in opposition to the force of the parking lock release spring to the initial position shown, and is held there by means of the lifting magnet or its retaining pin. The release device is then ready to be operated again. At the same time the actuating piston is moved in opposition to the force of the parking lock engagement spring in the direction toward the bearing and the parking lock is therefore released. Thus, the energy level of the engagement energy storage device in the form of the parking lock engagement spring is increased by means of the parking lock pressure.
In the above-described embodiment of the parking lock actuating device, the parking lock can be released by virtue of a corresponding actuating pressure. Below the necessary release pressure level, the parking lock can also be released by means of a storage spring, which is usually locked by an electromagnetic blocking magnet when de-energized, by appropriately energizing the locking magnet.
Disadvantageously, below the necessary pressure level, after being released by the spring the parking lock can no longer be engaged, since a corresponding pressure level is required for that. This means that the parking lock, which has been changed to its open operating condition by the spring, can only be engaged again by acting upon it appropriately with hydraulic pressure. If a corresponding functional defect occurs in the area of the hydraulic system or in the area of the electrical system that supplies current to the mechanical locking magnet, the parking lock is released or even not engaged to begin with, which in some circumstances, however, can give rise to safety-critical operating conditions of a vehicle made with the parking lock system.